An emulsion is defined as a macroscopic dispersion of two liquids--one of which forms the continuous phase of the system and the other forms the discontinuous or discrete phase. An emulsion of two liquids without a stabilizer will quickly break into two liquid layers.
Oil-in-water emulsions contain discrete droplets or particles of oil in a continuous water phase. Stability of such emulsions depends on the primary emulsifiers which emulsify and stabilize the oil droplets against coalescence. Secondary emulsifiers enhance the stabilizing property of the primary emulsifiers The secondary emulsifiers cannot by themselves emulsify and stabilize oil droplets against coalescence.
The use of the emulsion as a vehicle for skin preparations dates back to the invention of the first cold cream in about 150 A.D. The lotions or creams, in oil-in-water emulsion form, continue to this day to be the most popular delivery systems for applying functional cosmetics to the skin.
The effect of gravity on emulsions has long resulted in the general problem of sedimentation or creaming of the oil phase resulting in phase separation. It is a requirement of commercial emulsions that they should exhibit stability against such separation for extended periods of time, in some cases up to two years. Because of inability to cope with this handicap, many cosmetic products have failed at some stage of commercial development.
The prior art describes homopolymers of acrylic acids partially neutralized with long-chain alkylamines. Such homopolymers can function as primary emulsifiers or surfactants. Prior art emulsions prepared with a homopolymer of acrylic acid are not stable below pH of 5.7 when the homopolymer is neutralized with dodecylamine or below pH of 6.6 when the homopolymer is stabilized with n-hexylamine. It should be noted that pH of natural human skin is 5.5.